Mechanically induced intramolecular electron transfer in a mixed-valence molecular shuttle

The kinetics and thermodynamics of intramolecular electron transfer (IET) can be subjected to redox control in a bistable [2] rotaxane comprised of a dumbbell component containing an electron-rich 1,5-dioxynaphthalene (DNP) unit and an electron-poor phenylene-bridged bipyridinium (P-BIPY2+) unit and a cyclobis (paraquat-p-phenylene) (CBPQT(4+)) ring component. The [2] rotaxane exists in the ground-state co-conformation (GSCC) wherein the CBPQT(4+) ring encircles the DNP unit. Reduction of the CBPQT(4+) leads to the CBPQT(2(center dot+)) diradical dication while the P-BIPY2+ unit is reduced to its P-BIPY center dot+ radical cation. A radical-state co-conformation (RSCC) results from movement of the CBPQT(2(center dot+)) ring along the dumbbell to surround the P-BIPY center dot+ unit. This shuttling event induces IET to occur between the pyridinium redox centers of the P-BIPY center dot+ unit, a property which is absent between these redox centers in the free dumbbell and in the 1:1 complex formed between the CBPQT(2(center dot+)) ring and the radical cation of methyl-phenylene-viologen (MPV center dot+). Using electron paramagnetic resonance (EPR) spectroscopy, the process of IET was investigated by monitoring the line broadening at varying temperatures and determining the rate constant (k(ET) 1.33 x 10(7) s(-1)) and activation energy (Delta G double dagger = 1.01 kcal mol(-1)) for electron transfer. These values were compared to the corresponding values predicted, using the optical absorption spectra and Marcus-Hush theory.